全文获取类型
收费全文 | 3397篇 |
免费 | 274篇 |
国内免费 | 3篇 |
出版年
2021年 | 19篇 |
2020年 | 23篇 |
2019年 | 23篇 |
2018年 | 24篇 |
2017年 | 25篇 |
2016年 | 53篇 |
2015年 | 86篇 |
2014年 | 97篇 |
2013年 | 143篇 |
2012年 | 158篇 |
2011年 | 168篇 |
2010年 | 97篇 |
2009年 | 113篇 |
2008年 | 143篇 |
2007年 | 157篇 |
2006年 | 164篇 |
2005年 | 150篇 |
2004年 | 134篇 |
2003年 | 137篇 |
2002年 | 132篇 |
2001年 | 58篇 |
2000年 | 57篇 |
1999年 | 52篇 |
1998年 | 53篇 |
1997年 | 53篇 |
1996年 | 45篇 |
1995年 | 35篇 |
1994年 | 44篇 |
1993年 | 36篇 |
1992年 | 41篇 |
1991年 | 47篇 |
1990年 | 32篇 |
1989年 | 40篇 |
1988年 | 35篇 |
1987年 | 30篇 |
1986年 | 29篇 |
1985年 | 36篇 |
1984年 | 31篇 |
1983年 | 28篇 |
1982年 | 28篇 |
1981年 | 28篇 |
1980年 | 39篇 |
1977年 | 29篇 |
1975年 | 22篇 |
1974年 | 27篇 |
1973年 | 20篇 |
1970年 | 20篇 |
1968年 | 20篇 |
1936年 | 17篇 |
1932年 | 18篇 |
排序方式: 共有3674条查询结果,搜索用时 15 毫秒
101.
Ozone-induced changes of mRNA levels of β-1,3-glucanase,chitinase and ‘pathogenesis-related’ protein 1b in tobacco plants 总被引:4,自引:0,他引:4
D. Ernst M. Schraudner C. Langebartels H. Sandermann Jr. 《Plant molecular biology》1992,20(4):673-682
Treatment of the ozone-sensitive tobacco cultivar Bel W3 with an ozone pulse (0.15 l/l, 5 h) markedly increased the mRNA level of basic -1,3-glucanase and to a lower degree that of basic chitinase. The increase of -1,3-glucanase mRNA level occurred within 1 h and showed a transient maximum. Seventeen hours after ozone treatment, the -1,3-glucanase mRNA level decreased to lower values. The increase of basic chitinase mRNA level was delayed and was less pronounced than that of -1,3-glucanase mRNA. Cultivar Bel B showed only a small increase of -1,3-glucanase mRNA level after the same ozone treatment, whereas its basic chitinase mRNA was more strongly induced. Prolonged ozone treatment for 2 days of tobacco Bel W3 led to a persistent level of -1,3-glucanase and basic chitinase mRNAs, as well as to an increase of acidic chitinase and pathogenesis-related (PR) 1b mRNA levels. The results indicate that genes so far considered to code for PR proteins may also be involved in the plant response to oxidative stress. 相似文献
102.
We develop a model of the undulatory locomotion of nematodes, in particular that of Caenorhabditis elegans, based on mechanics. The model takes into account the most important forces acting on a moving worm and allows the computer simulation of a creeping nematode. These forces are produced by the interior pressure in the liquid-filled body cavity, the elasticity of the cuticle, the excitation of certain sets of muscles and the friction between the body and its support.
We propose that muscle excitation patterns can be generated by stretch receptor control. By solving numerically the equations of motion of the model of the nematode, we demonstrate that these muscle excitation patterns are suitable for the propulsion of the animal.
相似文献103.
104.
105.
106.
In a carrot (Daucus carota L.) cell line lacking the ability to undergo somatic embryogenasis, and in carrot and anise (Pimpinella anisum L.) cell lines in which embryogenesis could be regulated by presence or absence of 2,4-dichlorophen-oxyacetic acid (2,4-D), in the medium (+2,4-D=no embryogenesis,-2,4-D=embryo differentiation and development), the levels of endogenous gibberellin(s) (GA) were determined by the dwarfrice bioassay, and the metabolism of [3H]GA1 was followed. Embryos harvested after 14 d of subculture in-2,4-D had low levels (0.2–0.3 g g-1 dry weight) of polar GA (e.g. GA1-like), but much (3–22 times) higher levels of less-polar GA (GA4/7-like); GA1, GA4 and GA7 are native to these cultures. Conversely, the undifferentiated cells in a non-embryogenic strain, and proembryos of an embryogenic strain (+2,4-D) showed very high levels of polar GA (2.9–4.4 g g-1), and somewhat reduced levels of less-polar GA. Cultures of anise undergoing somatic embryo development (-2,4-D) metabolized [3H]GA1 very quickly, whereas proembryo cultures of anise (+2,4-D) metabolized [3H]GA1 slowly. The major metabolites of [3H]GA1 in anise were tentatively identified as GA8-glucoside (24%), GA8 (15%), GA1-glucoside (8%) and the 1(10)GA1-counterpart (2%). Thus, high levels of a GA1-like substance and a reduced ability to metabolize GA1 are correlated with the absence of embryo development, while lowered levels of GA1-like substance and a rapid metabolism of GA1 into GA8 and GA-conjugates are correlated with continued embryo development. Exogenous application of GA3 is known to reduce somatic embryogenesis in carrot cultures; GA4 was found to have the same effect in anise cultures. Thus, a role (albeit negative) in somatic embryogenesis for a polar, biologically active GA is implied.Abbreviations 2,4-D
2,4-dichlorophenoxyacetic acid
- GA
gibberellin(s) or gibberellin-like substances
- GC-RC
gas chromatography-radiochromatogram counting
- HPLC
high-presare liquid chromatography
- Rt
retention time
- TLC
thinlaver chromatography 相似文献
107.
Summary Regulation of passive potassium ion transport by the external calcium concentration and temperature was studied on cell cultures of 3T3 mouse cells and their DNA-virus transformed derivatives. Upon lowering of external calcium concentration, passive potassium efflux generally exhibits a sharp increase at about 0.1mm. The fraction of calcium-regulated potassium efflux is largely independent of temperature in the cases of the transformed cells, but shows a sharp increase for 3T3 cells upon increasing temperature above 32°C. In the same range of temperature, the 3T3 cells exhibit the phenomenon of high-temperature inactivation of the residual potassium efflux at 1mm external calcium. At comparable cellular growth densities, the transformed cell lines do not show high-temperature inactivation of residual potassium efflux. These results are consistent with the notion of a decisive role of the internal K+ concentration in the cell-density dependent regulation of cell proliferation. In particular, the growth-inhibiting effect of lowering the external Ca2+ concentrations is considered as largely due to a rise of passive K+ efflux and a subsequent decrease of internal K+ concentration. The experimental data on the Ca2+ dependence of passive K+ flux are quantitatively described by a theoretical model based on the constant field relations including negative surface charges on the external face of the membrane, which cooperatively bind Ca2+ ions and may concomitantly undergo a lateral redistribution. The present evidence is consistent with acidic phospholipids as representing these negative surface charges.This work is dedicated to the memory of Max Delbrück (deceased March 10, 1981), in whose laboratory in 1966 the earlier version of the present theoretical model was developed by one of the authors. 相似文献
108.
Ernst Weber Evangelos Papamokos Wolfram Bode Robert Huber Ikunoshin Kato Michael Laskowski 《Journal of molecular biology》1981,149(1):109-123
The third domain of Japanese quail ovomucoid, a Kazal type inhibitor, has been crystallized and its crystal structure determined at 2.5 Å resolution using multiple isomorphous replacement techniques. The asymmetric unit contains four molecules. In the crystal the molecules are arranged in two slightly different octamers with approximate D4 symmetry. The molecules are held together mainly by interactions of the N-terminal residues, which form a novel secondary structural element, a β-channel.The molecule is globular with approximate dimensions 35 Å × 27 Å × 19 Å. The secondary structural elements are a double-stranded anti-parallel β-sheet of residues Pro22 to Gly32 and an α-helix from Asn33 to Ser44. The reactive site Lys18-Asp19 is located in an exposed loop. It is close to Asn33 at the N terminus of the helical segment. The polypeptide chain folding of ovomucoid bears some resemblance to other inhibitors in the existence of an anti-parallel double strand following the reactive site loop. 相似文献
109.
Danny M. Gee Gary M. Hathaway Richard H. Palmieri Ernst A. Noltmann 《Journal of molecular biology》1980,142(1):29-42
As a corollary to X-ray crystallographic work performed by H. Muirhead, detailed studies on crystalline pig muscle phosphoglucose isomerase have been conducted to establish its basic physical and chemical properties. The enzyme species being investigated by X-ray diffraction has been determined to be isoenzyme III. Its molecular weight in the native state was found to be 132,000, its s020,w value to be 7·25 S. The enzyme is composed of two subunits of equal molecular weight (66,000). Its amino acid composition is largely similar to that of rabbit muscle phosphoglucose isomerase, with the significant exception that the pig muscle isomerase contains only three sulfhydryl groups per polypeptide chain (two of them accessible to titration with p-mercuribenzoate) as compared with twice that number for the rabbit muscle enzyme. This low number of sulfhydryl groups is interpreted as being responsible for the ease with which heavy-atom, isomorphous derivatives could be prepared for the pig muscle enzyme by Shaw & Muirhead (1977). 相似文献
110.
Edward Ernst 《BMJ (Clinical research ed.)》1980,281(6250):1280-1281